The present invention relates to the manufacture of gypsum board and in particular to a method of systematically reshaping the gypsum core or a portion of the gypsum core to produce a gypsum board having improved appearance and/or properties. The reshaping process results in core densification and can be used for many applications including producing end or cross tapers at the cut ends of the board, producing decorative patterns or textures in the surface of the board and for densifying the entire board core for special gypsum board applications.
Gypsum board is a laminate structure comprising a core of gypsum sandwiched between a face paper on one side and a back paper on the other side. Gypsum board is manufactured by a relatively high speed continuous method wherein a slurry of calcined gypsum and various additives are mixed with more than sufficient water for hydration and setting of the gypsum. The slurry is deposited on a lower, continuously advancing paper sheet and an upper continuously advancing paper sheet is layed over the slurry. The laminate structure is then formed into a continuous flat sheet of paper enclosed gypsum.
In the typical process the gypsum board is made face side down. The face paper, on the bottom, is folded upward along the two longitudinal edges and folded over onto the top of the slurry along these edges. The back paper is placed on top of the slurry, overlapping the edge portion of the face paper that is folded over onto the back side of the board. The continuous sheet is carried on a conveyor belt and rollers for a considerable distance until the gypsum core has set to a sufficient degree to permit the board to be cut into normal board lengths and transferred to high temperature drying kilns.
The bond between the paper and the gypsum core is of critical importance to the quality of the gypsum board. A poor quality gypsum board bond will result in a bond failure evidenced by the paper readily peeling away from the core with little force and no evidence or very light dusting of the gypsum core particles sticking to the paper surface. Another bond failure occurs with the paper separating from the core with various amounts or thicknesses of the core fragments adhering to the paper. This type of failure is referred to as a "split".
It has been the general belief in the industry that if the bond is disturbed during the gypsum setting process that a defect would result. Such defects are manifested in what are referred to as paper "blows" during the kiln heating in which bubbles or blisters form between the paper and core or "peelers" in which the paper peels cleanly from the core after drying without adhering to any of the gypsum.
One example within the industry of the concern with disturbing the bond involves the printing wheel used to label the boards. A printing wheel is typically used to label the back paper of the board before the continuous board is cut. If the pressure applied by the printing wheel exceeds a maximum value, a bond failure results. As a result, the printing wheel is closely monitored to avoid excess and/or imbalanced pressure application to the board causing this type of bond failure. Accordingly, it has been believed in the industry that any disturbance of the bond by pressure application during formation of the board will result in a bond failure.
Production of specialty gypsum board having a face surface other than substantially flat, except for an edge taper as discussed below, was thought to be impossible if the process involved the application of pressure. It was believed that pressure application to the board would destroy the gypsum board bond. The production of a decorative gypsum board was therefore limited to board produced with a decorative pattern printed on the face paper or produced by pre-embossing the paper prior to formation of the gypsum board. Both of these methods have their own drawbacks. For example, the use of a paper having a decorative printing or finish thereon can adversely affect the ability of water vapor to pass through the paper during drying of the gypsum board. With pre-printed or pre-embossed paper, the decorative patterns that can be used are limited to random patterns such that the boards do not have identical patterns.
Accordingly, it is an object of the present invention to develop a method of manufacturing gypsum board with a contoured face surface shape by using pressure without adversely affecting the gypsum core to paper bond.
It is a further object of the invention to produce the contoured or shaped gypsum board in an on-line process without significant reduction in the production rate of the gypsum board.
It has been found that the surface of the gypsum board core can be reshaped or contoured by a process of systematic pressure application to the gypsum core. The pressure application results in densification of the gypsum core and can take place at any time in the production process as long as the pressure application is controlled to produce only compressive loading on the gypsum core and no lateral shifting of the core mass occurs. Any shear stress at the paper/core interface or shear stress within the core that results in lateral displacement of the paper or the gypsum crystals destroys the bond, resulting in a bond failure. With compressive loading only, it has been found that if the bond is weakened by the pressure application, the bond ultimately heals such that after drying, there is a quality gypsum board bond. If shear stresses are induced resulting in a shift of the paper and/or gypsum crystals, the bond is completely destroyed and cannot be healed.
The setting of the gypsum core is an exothermic reaction resulting in a rise in temperature in the core. As a result, by monitoring the temperature of the gypsum core, the progress of the gypsum setting can be monitored. To avoid a lateral shift in the gypsum mass caused by pressure application, the hydration cycle must progress to a minimum point before the pressure can be successfully applied. The hydration cycle must reach the point where the core has attained a sufficient degree of stiffness to allow compression without the gypsum mass moving laterally. After the gypsum has reached this point, the densification can occur at any point up to and after the gypsum has reached its maximum temperature rise.
The unexpected finding that the gypsum core can be densified by the application of compressive loading was the result of an experiment conducted at a gypsum board production plant. The gypsum board, while setting and traveling on the conveyor belt, was simultaneously densified in two different manners. In the first case, a ten pound heavy aluminum pin was placed on the surface of the board and pressure was applied to create a continuous dent in the board surface as the board passed beneath the rotating pin. In the other case, pressure was applied to the board to create a depression of the same depth but the board was not allowed to pass under the applied load. Instead, the person applying the load walked with the moving board while exerting pressure at a single location. After drying, blisters and bond failures were found where the board was allowed to pass underneath the roller which was creating a drag between the paper and the core. The depression created through compressive force alone displayed a perfect paper to core bond.
The pressure applied is controlled within a predetermined range depending in part on the point in the gypsum hydration cycle where the pressure is applied. The compressive loading reshapes the gypsum core by densifying the gypsum by displacing gypsum crystals into the air voids formed in the gypsum core as well as into the voids left by evaporated water during the hydration cycle.
An example of pressure application to a board surface is found in U.S. Pat. Nos. 3,180,058 and 3,233,301 to Tillisch et al. There, a knurled roller was pressed in the board on the face surface along the edges to produce shallow discontinuous indentations in the board surface. The indentations are limited to the surface only and have depths of no more than 0.012 of an inch. The shallowness of the indentations is highlighted when it is compared to the current paper thickness of 0.016 of an inch. At the time of the Tillisch inventions the paper was likely thicker than it is today. The method of the present invention goes beyond the surface indentations formed by Tillisch to form relatively deep depressions by densifying the core.
In order to determine the effect of the Tillisch process on the board bond, the specifications of the Tillisch patents were followed in an experiment to evaluate the board bond. In the Tillisch patents it is noted that the indentations "do not affect the strength of the board edge." The effect of the Tillisch process on the bond itself however, is not mentioned in the patents. It was found that the areas of the board pressed by the projections of the knurled pin resulted in board bond failures while the bond in surrounding areas that were not pressed did not fail. Since the pressed areas were only one eighth inch square, significant area without bond failures remained, perhaps leading Tillisch to believe that the board bond was not effected and that the process was satisfactory.
It is believed that as pressure is applied with the knurled pin, the core material near the core surface is pushed laterally in front of the pin. This shear loading disrupts the bond forming between the paper and core and also disrupts the gypsum crystal structure resulting in bond failure. This is the same effect observed from the printing wheel if the pressure applied by the wheel exceeds a certain value or becomes imbalanced to create a drag between the core and paper. This test result emphasizes the need for a process in which a compressive force is applied without any or at least without significant shear forces being applied to the board. A principal cause of the shear stress in Tillisch is believed to be the failure to independently drive the knurled pin as well as the support wheel at line speed rather than letting the moving board rotate the pin. With a drive, the contact between the pin or roller surface and board can be static such that shear forces are substantially eliminated.
The core reshaping process can be used to produce a number of specialty gypsum boards. One application is the formation of a cross taper at the cut ends of the gypsum board. The ends of the board have not previously been tapered in a commercially viable process. Other applications include densifying the entire board for specialty applications and for producing a decorative shape or contour to the face of the gypsum board. The process will be described below primarily in the context of forming a board with end tapers.
Typical interior building construction comprises a plurality of spaced framing members referred to as studs, furring or joists. One or more layers of gypsum board are secured to one or each side of the framing members forming the wall or ceiling surfaces. The side edges of the gypsum boards are generally butted together over a framing member and nailed or screwed thereto with the fasteners extending through the gypsum board and into the framing members. To construct a monolithic appearing wall, the butt joints between adjacent gypsum boards are concealed by covering the joint with a reinforcing joint tape and several layers of a joint compound to cover the joint, the joint tape and the fasteners. To construct a smooth surface without ridges formed by the joint tape and compound, the gypsum board is produced with a slight taper on the face surface adjacent the longitudinal or side edges of the board. The taper results in a slight depression in the wall or ceiling surface at the joints. The depression is filled with the joint compound producing a smooth finish at the joint without a raised ridge.
As described above, the gypsum board is produced face down on a long conveyor as a continuous board that is later cut across its width into the desired length of board. It is common to produce a gypsum board with a taper at the longitudinal edges of the board parallel to the direction of board travel during manufacture. When the continuous board is produced, it is carried on a conveyor belt. Tapered edge belts are placed over the conveyor belt at the location of the two board edges so that the board is formed to the contour of the tapered edge belt. The tapering belts reduce the board thickness at the edges providing the depression for the joint tape and compound.
It is difficult however, to manufacture a gypsum board with a taper at the cut ends of the board, i.e., the ends of the board transverse to the direction of board travel during production. As a result, when the cut ends of the gypsum board are used to form a butt joint, there is no taper into which the fasteners, reinforcing joint tape and joint compound can be concealed. With a butt joint without tapers in the gypsum board, it is necessary to feather, or thin, the joint compound over a considerable width on both sides of the joint in an effort to conceal it. However, under certain lighting conditions this raised ridge at the joint can be detectable.
This problem could be overcome in six to twelve foot wall or ceiling sections by installing the gypsum board parallel to framing members. However, due to the orientation of the surfacing paper fibers it is more desirable to install the board at right angles to the framing for strength and sag resistance. Perpendicular application often creates the condition of abutting end joints. With an end taper however, abutting end joints can easily be made without forming a ridge of tape and joint compound.
Attempts have been made in the past to produce tapered areas across the width of a board at the desired length intervals during the board production by placing cross tapering belts or slats between the board and the main conveyor belt. This method presents several problems, however, which have prevented successful commercialization. One problem is material management, i.e. what to do with the gypsum displaced by the cross belt. The slurry is discharged onto the face paper at a constant rate. If the amount of material needed at a particular location is reduced by the cross belt, the excess material must have some place to go. Another problem is in synchronizing the tapers with the knife used to cut the continuous board into individual boards. Expansion of the board during the hydration of the gypsum slurry and slippage of the board over the conveyor belt have made it difficult to accurately synchronize the cross tapers with the knife cuts.
As a result, there has been no commercially viable method developed to form an end taper in a gypsum board with an on-line process. One attempt to produce an end taper off-line has been to physically remove a portion of the gypsum core by cutting into the board parallel to the board face with a saw blade. After a portion of the core has been removed by the saw blade, the thin layer of gypsum material remaining on the face paper is bent inward, closing the saw cut groove and resulting in a taper in the face surface of the board. Such a tapering operation, however, significantly reduces the strength of the board at the critical location where the board is fastened to the framing members. In addition, the method is time consuming and must be performed off-line, resulting in significant added cost.
Other methods have been proposed such as removing the face paper and a portion of the underlying gypsum core along the cut ends, providing a depression to fill with the joint compound. This method however, cannot be used with joint tape. The width of the removed face paper and core must be narrower than the width by which the gypsum board overlaps the framing members so that the fasteners can be placed in the face paper rather than in the area where the paper has been removed. The resulting width of the removed board portion is narrower than the reinforcing joint tape making use of joint tape impractical. If the paper is removed from an area wide enough to accommodate the joint tape, it will be too weak to withstand handling and the nail holding power will be substantially decreased.
There is a tapered end gypsum board available in the European market. The tapers are accomplished by again, removing a portion of the face paper at the board end and machining the taper in the gypsum core. With this board, joint tape is not used to finish the gypsum board. Instead, a specially formulated joint compound is filled in the depression. To use a joint tape, a wider portion of the paper would need to be removed which will pose the same problems with nail holding and strength as described above. This tapeless joint system is another example of the need and the attempts by the industry to try to create a taper at the board ends. The use of the core reshaping process of the present invention to produce an end taper in the board during on-line board production satisfies this need in the industry in a commercially viable manner.
Another advantage of end tapers produced by core densification is a reduced drying rate of the gypsum core at the cut ends. Air flowing over a board in the dryer has a tendency to dry the board faster at the periphery of the board. This is more pronounced at the cut ends than the finished edges due to impingement of the hot dryer air directly on the board ends. The result can be overdrying of the gypsum at the cut ends. By densifying the core at the cut ends, the rate of drying is reduced such that overdrying can be avoided.
Another advantage of tapered ends is that by now enabling end to end butt joints to be made smoothly, without a hump, the board can now be easily installed perpendicular to the wall framing members. This can shorten to total linear length of joints by using boards longer than eight feet and also positions the majority of the joint at the four foot level where it can be more easily finished. Perpendicular installation also reduces sagging of the board as discussed above.
Core reshaping can be accomplished at any point in the production cycle after the core has set sufficiently to provide enough stiffness to allow compression without the gypsum moving in the lateral direction. There are, however, preferred locations in the process that are better suited to accomplishing core reshaping. Reshaping the core early in the gypsum hydration cycle has advantage of lowering the force requirement. However, the memory retention capability of the core is lower in part due to the gravitational pull on the core. For end tapers or other contouring, the effect of gravity is of particular concern because the board is traveling face down and the contour or end taper is pressed upwardly into the board resulting in no support immediately below the contoured face surface. Reshaping the core later in the hydration cycle, i.e. closer to the knife, would reduce the effect of gravity but would increase the amount of force needed to densify the core. The preferred time for reshaping is at about 40 to 45 percent of the gypsum hydration cycle.
Later in the board production cycle the board is turned face up before it enters the dryer. After the board has been inverted and before it enters the dryer is another opportunity for core reshaping. At this stage, normally 90 percent or more of the hydration has occurred.
Besides the production of an end taper, another application of the reshaping process is the production of gypsum board having a contoured or patterned surface. Such gypsum board has been previously produced by an off-line pressing operation after the board has been dried. However, the process typically results in a "split" in the gypsum core. The process of this invention allows such a pattern to be pressed into the gypsum board by systematically densifying the core before the board enters the dryer without adversely affecting the board bond, thereby producing a high quality product.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.